Differential Input, Differential Output. I recommend the slowest simulation speed to watch as the tail current switches between the two branches of the diff amp.
The two inputs share the same DC level (Common Mode DC level). The AC signal is shifted by 180degrees between the two inputs which is the same as multiplying one of the inputs by minus one. These are then referred to as differential signals and there are many cool ways of removing noise from your signals with them.
The output is measured across the two branches and is referred to as a differential output. If I measured just one branch relative to ground it would be a single ended output.
